Endocrine system acts with nervous system to coordinate and integrate activity of body cells • Influences metabolic activities via hormones transported in blood

Nervous system: Initiates responses rapidly, Short duration responses, Acts via action potentials and neurotransmitters, Acts at specific locations determined by axon pathways, Neurotransmitters act over very short distances.

Endocrine system: Initiates responses slowly, Long duration responses, Acts via hormones released into the blood, Acts at diffuse locations targets can be anywhere blood reaches, Hormones act over long distances.

The endocrine and nervous systems interact to regulate internal systems and respond to external stimuli

Maintains and regulates internal homeostasis • Water and nutrient balance • Metabolism • Immune response • Growth and development (mitosis) • Reproduction (meiosis, maturation) • Behaviour

Endocrine cells→ Endocrine glands→ Endocrine organs • Ductless glands that release secretions (hormones) into surrounding body fluids (interstitial fluid, lymph system, and blood stream)

Several organs act primarily as endocrine glands - Pineal glands, Pituitary gland, Thyroid and Parathyroid glands, Adrenal glands,

Several organs also act as endocrine glands - Hypothalamus - Thymus - Pancreas - Gonads

Other organs and tissues have endocrine cells embedded in their structure and signal via the endocrine system

Long-distance chemical signals. There are a large variety of hormones: • Amino-acid based hormones • Amino-acid derivatives (amino acid plus something else) • Peptide hormones and protein hormones (encoded in DNA and expressed like all proteins)

They are lipophobic/hydrophilic, meaning they are membrane impermeable (except thyroid hormone) • They can be synthesized and stored by endocrine cells for immediate release • Target cells must have extracellular hormone receptor • Act through second-messenger systems (e.g., cAMP)

Hormone receptors expressed by target cells act via intracellular secondmessenger pathways - '-protein mediated pathways - Cyclic Adenosine Monophosphate (cAMP) pathway - Cyclic Guanosine Monophosphate (cGMP) pathway - Receptor-coupled kinase

cGMP pathway works similar, just GTP replaces ATP

Second messenger pathways allows for the amplification of the initial signal (hormone) by multiplying the response in each stage of subsequent activation and control at each level of signalling • Temporal (when) • Spatial (where) • Quantitative (how much) • Qualitative (what)

Long-distance chemical signals. There are a large variety of hormones: • Amino-acid based hormones • Amino-acid derivatives (amino acid plus something else) • Peptide hormones and protein hormones (encoded in DNA and expressed like all proteins) Endocrine System – Hormones • Adrenalin - Stress hormone and neurotransmitter acting on various organ systems • Serotonin – Hormone and neurotransmitter (“happiness” transmitter) that effects the cardiovascular system • Melatonin - Hormone and neurotransmitter that helps regulate our circadian “inner” clock

• Growth hormone- controls growth and development • Antidiuretic hormone (ADH) - regulates water reabsorption by the kidneys • Insulin - lowers blood sugar level • Glucagon – increases blood sugar level

Long-distance chemical signals. There are a large variety of hormones: • Steroid based hormones • All synthesized from cholesterol precursor • DNA encodes enzymes for hormone synthesis, but not structure • They are lipid-soluble (lipophilic/hydrophobic), meaning they are membrane permeable • Not stored in endocrine cells, created on demand • They can enter the cell and act on intracellular receptors to directly activate genes • Often synthesized and released as inactive precursor, requiring modification by target cells (enzymes required to activate hormone)

Steroid hormones regulate: - Metabolism - Inflammation and the immune system - Water balance and blood pressure - Development and growth, sexual maturation and reproduction

Release of hormones is mostly regulated by negative feedback • Bring an internal condition or signal back to homeostasis by stimulating a physiological response

The production and release of hormones is controlled by several factors: - Humoral stimuli - a change of ions or nutrients in the body fluids is detected by receptors of the endocrine cells and triggers the release of appropriate hormones

The production and release of hormones is controlled by several factors: - Neural stimuli - endocrine cells are stimulated by neurons (e.g. stress response = adrenalin release)

The production and release of hormones is controlled by several factors: - Hormonal stimuli - Endocrine cells respond to hormones released by other endocrine cells

Binding of a hormone triggers a cellular response and the nature of that response is cell is cell specific not hormone specific • One hormone can have many different effects depending on the target cell Response strength depends on : 1. Hormone concentration (titer) – hormones work at very low concentrations 2. Density of receptors on the target cell 3. Affinity of a given receptor for specific hormones

At any given time many hormones circulate in our body. Target cells are usually influenced by more than one hormone at a time • Control of the endocrine system is complex and somewhat unpredictable

Signal regulation and hormone interactions fall into broad categories: - Permissiveness - One hormone needs the presence of another hormone to exert its full effect. This allows for fine-scale temporal and special control (1+1=1)

- Synergism - More than one hormone produces the same effect in a target cell. More than one type of hormone will amplify the effect (1+1=3)

- Antagonism - Different hormones oppose each other’s actions. The response from the target cell depends on the hormone-hormone interaction (1+1=0)